1. Field of the Invention
The present invention generally relates to the field of wireless communications and specifically relates to techniques for implementing an efficient combination of various MIMO (Multiple Input Multiple Output) transmission control techniques and transmission channels, taking into account the state of each physical channel in downlink.
2. Description of the Related Art
HSDPA (High Speed Downlink Packet Access), which has been standardized by 3GPP, provides a maximum transmission speed of 14.4 Mbps, leading to a gradual introduction of high speed and large capacity mobile communications. However, widespread use of mobile terminals including mobile phones and of the Internet, as well as an increase in variety and sophistication of the content is leading to demands for increased capacity and frequency utilization and optimized IP traffic.
LTE (Long Term Evolution), which is now being put into active use, assumes a maximum downlink transmission speed of 100 Mbps. Moreover, optimization is being demanded for both a low-speed moving terminal and a high-speed moving terminal.
With MIMO transmission, different signals are transmitted over parallel transmission paths formed by multiple inputs (transmit antennas) and multiple outputs (receive antennas) (MIMO multiplexing). This technique is considered likely to become a mandatory technique for the LTE since speed can be increased in proportion to the number of the parallel transmission paths even though the same frequency is used.
As a wireless access scheme, OFDM (orthogonal frequency division multiplexing) is suitable for high-speed transmission at the speed of over several tens of Mbps. In the OFDM, which utilizes orthogonality of frequencies, sub-carrier spectra are arranged in high density so that they overlap one another, increasing the frequency utilization. In a transmission using n sub-carriers, as a signal is divided over the multiple sub-carriers, the symbol length becomes n times relative to a system which sends a signal in one carrier.
Another technique is being proposed such that, in an MIMO-OFDM transmission scheme, which spatially multiplexes OFDM signals, phase-hopping transmit diversity is performed, which applies a different phase rotation in each sub-carrier and transmit antenna such that the transmit signals are orthogonal to one another when received, increasing the transmission rate in proportion to an increase in the number of transmit antennas due to the spatial multiplexing (e.g., see Patent document 1).    Patent Document 1:    JP2006-081131A